Oxysterols form a large family of oxygenated derivatives of cholesterol that are present in the circulation, and in human and animal tissues. Oxysterols that have been identified in human plasma to date include 7α-hydroxycholesterol, 24S-hydroxycholesterol, and 4α- and 4β-hydroxycholesterol, which are present at concentrations ranging from 5-500 ng/ml. These oxysterols have a variety of half-lives in circulation ranging from 0.5-60 hours, and their levels can be altered by aging, drug interventions, and disease processes. Oxysterols may be formed either by autooxidation, as a secondary byproduct of lipid peroxidation, or by the action of specific monooxygenases, most of which are members of the cytochrome P450 family of enzymes. Examples of these enzymes are cholesterol 7α-hydroxylase (CYP7A1) that forms 7α-hydroxycholesterol, cholesterol 25-hydroxylase that forms 25-hydroxycholesterol, cholesterol 24S-hydroxylase (CYP46) that forms 24S-hydroxycholesterol, and others. In addition, oxysterols may be derived from the diet. Cytochrome P450 enzymes are also involved in the further oxidation of oxysterols and their metabolism into active or inactive metabolites that leads to their eventual removal from the system. Certain oxysterols can have potent effects on cholesterol metabolism. Oxysterols have been found to be present in atherosclerotic lesions. Oxysterols may play a role in various physiologic processes, such as cellular differentiation, inflammation, apoptosis, and steroid production.
Osteoporosis and its complications cause morbidity and mortality in the aging population, and can result from increased bone resorption by osteoclasts in parallel with decreased bone formation by osteoblasts.